The present invention relates to the field of radiography. It finds particular application in conjunction with the x-ray tubes for computed tomographic scanners and will be described with particular reference thereto. However, it is to be appreciated that the invention may find further application in other areas of radiography, such as medical diagnostic digital x-ray, conventional x-ray, radiation therapy, and the like.
In computed tomography, a slice of a patient to be examined is disposed in a scan circle of the scanner. A fan shaped x-ray beam is projected from an x-ray tube through a shutter, a collimator, the scan circle, and the patient slice to an array of radiation detectors. By rotating the radiation source, shutter, and collimator relative to the patient, radiation is projected through the imaged slice to the detectors from a multiplicity of directions. From radiation intensity data sampled at the detectors, data indicative of the path the sampled radiation traveled to reach each sampled detector, and other data, an image of the examined slice of the patient is reconstructed.
One of the problems encountered in CT scanners is the deleterious effect of off-focal radiation. In the x-ray tube, an electron beam strikes a focal spot point or line on an anode. X-rays are generated at this focal spot and travel along diverging linear paths in an x-ray fan beam of dimension controlled by the collimator. If all the radiation were emitted from the focal spot, then the path traveled by each x-ray beam from the x-ray tube to the detector for each detector sampling could be accurately determined. However, x-rays are emitted from regions of the anode other than the focal spot. In CT scanning x-ray tubes, 3% to 8% of the detected radiation is commonly off-focal radiation, i.e. radiation not originating at the focal spot. The spread in the origin of the radiation from off-focal radiation causes small objects and sharp edges to lose proper definition and become blurred. The lack of definition gives rise to non-linear artifacts in the reconstructed image.
The CT scanner collimators are commonly disposed adjacent the scan circle, i.e. displaced from the x-ray tube anode. Although this placement assures accurate beam dimensions in the scan circle, off-focal radiation originating on portions of the anode well displaced from the focal spot are permitted to pass through the collimator to the detectors. The greater the distance between the anode and the collimator, the greater the parallax and off-focal radiation may originate more distantly from the focal spot and still pass through the collimator to the detectors, i.e. the more blurred the focal spot becomes.
The off-focal radiation has a particularly deleterious effect on the bone correction applied to brain scans. A calcium correction is commonly made to minimize the effects attributable to radiation spectrum changes due to absorption by the bone tissue. The calcium correction deconvolves the effect of the broad spectrum radiation source's projection across the bone/brain interface on the soft tissue. The variety of sizes, shapes, and density of skulls which may be examined in common clinical practice render impractical a universal correction for the effects of off-focal radiation and beam hardening.
The present invention includes a method and apparatus for reducing off-focal radiation for head scans without affecting whole body scans.